Electromagnetic Stirring for Silicon Crystallization Purity
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Solution Overview
Problem
Existing methods for crystallizing silicon for photovoltaic cells do not achieve optimal purity and unidirectional solidification, leading to impurities in the silicon ingots and uneven solidification fronts, which affect the performance of photovoltaic cells.
Innovation Solution
A method involving a crucible with a heating and heat dissipating arrangement and an electromagnetic stirring device, where the stirring speed is dynamically controlled in response to the solidification rate, with multiple stages of stirring to ensure a clean and unidirectional solidification front, and the ability to alter stirring direction for improved temperature homogeneity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If the stirring speed is increased to remove impurities, then the purity of silicon is improved, but the solidification front becomes unstable and unidirectional growth is compromised
Solution Approach 1:
The stirring speed is dynamically adjusted during the crystallization process. High stirring speeds are applied initially to remove impurities from the melt, then the speed is reduced as solidification progresses to maintain a stable unidirectional solidification front. This dynamic control resolves the contradiction between impurity removal and solidification stability.
Solution Approach 2:
Intense stirring is applied in the preliminary stages of crystallization when the melt is fully liquid, to maximize impurity removal before solidification begins. This preliminary impurity removal action allows subsequent solidification to proceed with a cleaner melt, reducing the need for high stirring speeds that would destabilize the solidification front.
2Manufacturing precision
If the stirring is maintained throughout solidification to keep silicon pure, then impurity removal is improved, but remixing of impurities at the solidification front occurs
Solution Approach 1:
The stirring speed is dynamically reduced as solidification progresses. When the solidification front approaches, the stirring speed is lowered to prevent the mechanical disturbance from remixing impurities that have been segregated to the solidification front. This dynamic adjustment maintains purity without causing remixing.
Solution Approach 2:
The stirring is applied in periodic cycles rather than continuously. Stirring is intensified during liquid phases to promote impurity removal, then reduced or paused during critical solidification phases to prevent remixing. This periodic action pattern optimizes both impurity removal and prevents harmful remixing.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach results in higher purity silicon with a clean solidification front, avoiding impurity remixing and ensuring a unidirectional growth, thereby enhancing the quality and performance of photovoltaic cells.
Implementation Method 1
an electromagnetic stirring device supplied with an alternating current for applying an alternating electromagnetic field to the crucible
Implementation Method 2
a heating device for heating the crucible
Implementation Method 3
a heat discharging device for discharging the heat from the crucible
Data Source
Figure 1~2
Figure 3~4
AI summary
An apparatus for crystallization of silicon comprises a crucible (11) for containing silicon, a heating and heat dissipating arrangement (12) provided for melting the silicon contained in the crucible and for subsequently solidifying the molten silicon, and an electromagnetic stirring device (13) provided for stirring the molten silicon in the crucible during the solidification of the molten silicon. A control arrangement (14) is provided for controlling the heating and heat dissipating arrangement to solidify the molten silicon at a specified solidification rate and for controlling the electromagnetic stirring device to stir the molten silicon in response to the specified solidification rate of the molten silicon such that the ratio of a speed of the molten silicon and the specified solidification rate is above a first threshold value.